Exophthalmos is generally understood to be the anterior displacement of the eyeball in relation to the cranium. Exophthalmos which can be unilateral or bilateral, may occur in a variety of different conditions; for example, endocrine imbalances, in particular hyperthyroidism or Graves' disease; intraorbital tumors; and, various space occupying processes in the orbit.
Exophthalmos can be measured with instrumental help using an exophthalmometer. The exophthalmometer is used to record any of three categories of exophthalmos. The first is absolute, where the measurement is compared with a known normal value. The second is relative exophthalmos, where a comparison is made of the measurement between the two eyes in the same individual, and the third is comparative exophthalmos where measurements in the same individual are compared from time to time. The degree of exophthalmos is generally measured by determining the extent of protrusion of the corneal apex from some reference point on the cranium. Most of the known exophthalmometers have employed the lateral orbital rim (i.e. outside bony wall of the eye socket) as a reference point and measure in an anterior-posterior plane the distance between the lateral orbital rim to the corneal apex with the eyes looking straight ahead.
The Hertel exophthalmometer (Hertel, E., Arch. F. Ophth. 60:171, 1905) is the most commonly used measurement instrument since it is cost-effective and relatively easy to learn and apply. Large population studies using the Hertel have provided normal range statistics for protrusion of the eyes and over time these have become the standard in orbital disease literature and an integral part of orbital examination. (For example, see Glaser, J. S., Orbital Disease and Neuro-Ophthalmoloy. In Tasman, W., and Jaeger, E. A., (Ed): Duane's Clinical Opthalmology; Vol. 2, Ch. 14, p. 6-7; J. B. Lippincott, Philadelphia, 1990; Young, I. S., and Henkind, P.: Medical Aspects of Graves' Opthalmopathy, In Smith, B. C., (Ed): Ophthalmic Plastic and Reconstructive Surgery; Vol. 2, Ch. 75, p. 1364, C. V. Mosby, St. Louis 1987; and Rootman, J.: Pathophysiologic Approach to Clinical Analysis of Orbital Disease, In Rootman, J., (Ed): Diseases of the Orbit, Ch. 6, p. 84, J. B. Lippincott, Philadelphia, 1988).
The Hertel exophthalmometer is a binocular instrument which rests on each lateral orbital rim and allows an observer in front, with the aid of mirrors to view images of the corneal apex of the two eyes as seen in profile superimposed upon a millimeter scale. A measurement is obtained of the relative distance of the apex of the cornea from a zero reference point, i.e., an imaginary horizontal line in a plane parallel to the front of the patient's face uniting the lateral orbital rims.
Several instruments with reference points other than the lateral orbital rims have been proposed (See Drews, 1956, Trans. Am. Ophthl. Soc. 54:215) but none of these instruments are in general use. For example, superior and inferior orbital margins have been proposed as reference points by Cohn (Jb. d. Schles. Ges. F. Enterl. Kultur 43:156, 1865; Klin. Mbl. Augenbeilk 5:339, 1867; and It'I Int. Cong. Ophthal. Paris 21, 1867), Landolt (1874), Mutch (Brit. J. Ophthal. 23:677, 1939) and Naugle (Proc. VIIth Congress European Soc. Ophthal. Helsinki 549, 1985). Watson in 1967 (Trans. Ophthal. Soc. U.K. 87;409) designed a device to fit over the nose and orbital tubercles.
Instruments, such as the Hertel exophthalmometer, which measure from the lateral orbital rims, are not useful for measuring exophthalmos in cases such as some endocrine exophthalmos where tissue edema can cause extensive swelling in the periorbital soft tissues or in all cases where there has been any disease, trauma, surgery or congenital anomaly affecting the lateral orbital rims. For example, the Hertel can not be used post-operatively in the case of a lateral orbitotomy which is used routinely for the removal of orbit tumors and Graves' orbital surgery.
In view of the above-mentioned limitations in exophthalmometers, stereophotogrammetric and radiographic techniques have been developed. (Beard, L. F. et al; A Straightforward Approach to Stereometric Photography for Medical Purposes, p. 27-48, In Herron, R. E. (ed). Biostereometrics 74, Falls Church, Va., and Backlund E. O. and Torlegard, K., Acta Opthalmol. (Copenh) 1968; 46(3): 575-579; Tengroth, B., Acta. Ophthalmol (Copenh) 1964); 42(4): 855-863; Borgen, H. G. et al., Trans. Am. Acad. Opthalmol. Otolaryngol, 1976, March-April, 81(2): 298-304; and Bogren, H., and Tengroth, B.; Clin. Radiol. 1967 April: 18(2); 193-196). The drawback to these techniques is that they require lengthy evaluation procedures and complex instrumentation.